The present invention is related to audio signal processing and particularly to audio signal processing in the context of speech coding using adaptive bass post-filters.
Bass post-filter is a post-processing of the decoded signal used in some speech coders. The post-processing is illustrated in FIG. 11 and is equivalent to subtracting from the decoded signal ŝ(n) a long-term prediction error which is scaled and then low-pass filtered. The transfer function of the long-term prediction filter is given by:
            P      LT        ⁡          (      z      )        =      1    -                  1        2            ⁢              z                  -          T                      -                  1        2            ⁢              z                  +          T                    where T is a delay which usually corresponds to the pitch of the speech or the main period of the pseudo-stationary decoded signal. The delay T is usually deduced from the decoded signal or from the information contained directly within the bitstream. It is usually the long-term prediction delay parameter already used for decoding the signal. It can also be computed on the decoded signal by performing a long-term prediction analysis. The post-filtered decoded signal is then equal to:(n)=ŝ(n)−α(ŝ(n)*pLT(n)*hLP(n))where α is a multiplicative gain corresponding to the attenuation factor of the anti-harmonic components and hLP(n) is the impulse response of a low-pass filter. As for the delay T, the gain can come from directly the bitstream or computed form the decoded signal.
The bass post-filter was designed for enhancing the quality of clean speech but can create unexpected artifacts which can spoil the listening experience, especially when the anti-harmonic components are useful components in the original signal, as it can be the case for music or noisy speech. One solution of this problem can be found in [3], where the post-filter can be by-passed thanks to a decision determined either at the decoder side or at the encoder side. In the latest case, the decision needs to be transmitted within the bitstream as it is depicted in FIG. 12.
In particular, FIGS. 11 and 12 illustrate a decoder 1100 for decoding an audio signal encoded within a bitstream to obtain a decoded signal. The decoded signal is subjected to a delay in a delay stage 1102 and forwarded to a subtractor 1112. Furthermore, the decoded audio signal is input into a long-term prediction filter indicated by PLT(z). The output of the filter 1104 is input into a gain stage 1108 and the output of the gain stage 1108 is input into a low-pass filter 1106. The long-term prediction filter 1104 is controlled by a delay T and the gain stage 1108 is controlled by a gain α. The delay T is the pitch delay and the gain α is the pitch gain. Both values are decoded/retrieved by block 1110. Typically, the pitch gain and the pitch delay are additionally used by the decoder 1100 to generate a decoded signal such as a decoded speech signal.
FIG. 12 additionally has the decoder decision block 1200 and a switch 1202 in order to either use the bass post-filter or not. The bass post-filter is generally indicated by 1114 in FIG. 11 and FIG. 12.
It has been found that controlling the bass post-filter by the pitch information such as the pitch gain and the pitch delay or the complete deactivation of the bass post-filter are not optimum solutions. Instead, the bass post-filter can enhance the audio quality substantively if the bass post-filter is correctly set. On the other hand, the bass post-filter can seriously degrade the audio quality, when the bass post-filter is not controlled to have an optimum bass post-filter characteristic.